Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a charcoal canister. During a subsequent engine operation, the stored vapors can be purged into the engine where they are combusted. In addition to canister fuel vapors, positive crankcase ventilation fuel vapors may also be ingested and combusted in the engine during engine operation.
One common issue with the purging of crankcase and canister hydrocarbons to an engine intake is the control of a combustion air-fuel ratio. In particular, due to large discrepancies in the estimation of fuel vapor concentrations from the canister and the crankcase, it may be difficult to control the air-fuel ratio of the cylinders where the vapors are introduced for combustion. As such, the air fuel ratio errors can lead to degraded engine performance and elevated exhaust emissions.
The inventors herein have recognized that more reliable air-fuel ratio control can be achieved during purging in engine systems configured with a sole cylinder that is dedicated for providing external EGR to other engine cylinders. In particular, engine systems with a dedicated EGR cylinder may be configured to operate the dedicated cylinder providing the EGR richer than stoichiometry while adjusting fueling of the non-dedicated cylinders (that is, the remaining engine cylinders) to maintain an overall stoichiometric exhaust. As a result, the dedicated EGR cylinder may have a higher tolerance for deviations from a desired air fuel ratio. Further, there may be multiple opportunities for accurately estimating and addressing air-fuel ratio deviations at both the EGR cylinder as well as the non-dedicated EGR cylinders. For example, a first air-fuel ratio sensor coupled to the dedicated EGR cylinder may enable air-fuel ratio deviations arising at the dedicated EGR cylinder (such as due to the purging of fuel vapors to the dedicated EGR cylinder) to be estimated and corrected for. In addition, air-fuel ratio deviations arising at the non-dedicated EGR cylinders due to recirculation of exhaust gas from the dedicated EGR cylinder can be better estimated and compensated for based on the output of the first air-fuel ratio sensor. Further still, air-fuel ratio deviations can be estimated based on the output of a second air-fuel ratio sensor coupled to the non-dedicated EGR cylinders and used to correct the fueling of both the dedicated EGR cylinder as well the remaining engine cylinders. Consequently, more accurate air fuel ratio control can be achieved during purging conditions by allowing the dedicated EGR cylinder to be enriched with at least the purge vapors, while the engine air fuel ratio is controlled more strictly at the remaining cylinders.
Thus, in one example, purge control is improved by a method comprising selectively purging fuel vapors from one or more of a fuel system canister and a crankcase to enrich only a dedicated cylinder group of a multi-cylinder engine and recirculating exhaust gas from the dedicated cylinder group to each of remaining non-dedicated EGR engine cylinders and the dedicated cylinder group. In this way, air-fuel ratio control during purging is improved.
As an example, in response to purging conditions being met, fuel vapors from a fuel system purge canister as well as from crankcase ventilation may be purged to a single dedicated EGR cylinder of a multi-cylinder engine. Based on the purge rate, fueling of the dedicated EGR cylinder may be adjusted so that the cylinder is operated richer than stoichiometry. As such, the purge content received in the dedicated EGR cylinder may be feed-forward estimated based on the canister load, purge rate, etc. The rich exhaust from the cylinder may be passed through a water gas shift (WGS) catalyst coupled downstream of the cylinder for the purpose of creating hydrogen from the hydrocarbons in the rich exhaust. Hydrogen enriched exhaust from the dedicated EGR cylinder is then recirculated via an EGR passage to all the engine cylinders. An air-fuel ratio of the hydrogen enriched EGR received in the engine cylinders may be estimated based on the output of an air-fuel ratio sensor coupled downstream of the EGR donating cylinder. Fueling of the non-dedicated cylinders is then adjusted based on the air-fuel ratio of the received EGR so as to maintain stoichiometric combustion.
In this way, stoichiometric air-fuel ratio control is enabled in non-dedicated EGR cylinders without requiring accurate estimation of purge content. By selectively delivering purge fuel vapors to a dedicated EGR cylinder, at least a portion of the cylinder enrichment may be provided by purge vapors, improving fuel usage. By delivering hydrogen enriched EGR from the purge vapor receiving cylinder to all or only the non-dedicated engine cylinders, combustion stability of a highly EGR diluted engine is improved, allowing the engine to operate more efficiently.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.